Thermoplastic vulcanizate with defined morphology for optimum elastic recovery

ABSTRACT

A thermoplastic vulcanizate (TPV) is produced by melt blending until the volume fraction of dispersed rubber particles is greater than 0.5 preferably from 0.55 to 0.95, the remaining being the dispersed continuous plastic phase in a manner so as to develop a morphology which generates optimum elastic recovery. The majority (by volume) of large rubber particles are adjacent a small particle which is separated from one or more large particles by a critically thin, less than 0.1 $G(m)m thick, ligament of deformable plastic. When the majority of large particles evidence this morphology, as seen in a transmission electron microscope (TEM) photomicrograph, the TPV exhibits unexpectedly higher elastic recovery than if the ligaments were thicker. The foregoing is simulated in a micromechanical model which confirms, and in retrospect, predicts the observed actual elastic recovery of the TPV.

CROSS REFERENCE TO RELATED APPLICATION

This application is filed pursuant to Provisional Application No.60/144,362 filed on July 16, 1999.

FIELD OF THE INVENTION

Polymer blends having a combination of elastic and thermoplasticproperties, referred to as “thermoplastic vulcanizates” or “TPVs” (alsoreferred to in the past as “thermoplastic elastomers” or “TPEs”) aremade by dynamic vulcanization to provide desired hardness/softness, oiland temperature resistance, oxidation resistance, and processability,inter alia. In thermoplastic elastomers which are elastomeric alloys andnot physical blends, the properties depend on the relative amounts of“hard” and “soft” phases provided by each component, and the propertiesof each component. To be of commercial value, the hard phase istypically provided by a readily available engineering thermoplasticresin, familiarly referred to as a “plastic” for brevity. Most commonlythe plastic is chosen from polyesters, polyamides and polyolefins whichprovide a continuous phase of the hard phase in which dispersed domainsof the “soft” phase of an elastomer are present. Optimizing the elasticrecovery of a TPV and confirming the physical nature of its definedmorphology, is the subject of this, invention. Confirmation is obtainedwith both photomicrographs and computer modelling. The photomicrographsare from an electron microscope, preferably a transmission electronmicroscope (TEM) Of particular interest are relatively “soft” blends ofa vulcanizable (hereafter “curable” for brevity) rubber havingcontrolled hardness less than about 90 Shore A. Such blends areexceptionally resistant to oil swelling, and to compression set. Theterm “elastomer” is used herein to refer to a vulcanized blend ofpolyolefin and rubber which may be formulated to exhibit varying degreesof elasticity such that a test strip 2.5 cm wide and 25 mm thick may bestretched in the range from about 5% to 100% of its initial length andstill return to it; further, such vulcanized elastomer is necessarilythermoplastic and re-processable.

The Problem

There is a market need for blends of polar engineering thermoplasticscontaining a dispersed “polar rubber” phase and a continuous “plastic”phase, which blends have high elastic recovery. The term “elasticrecovery” refers to the proportion of recovery after deformation and isquantified as percent recovery after compression. A TPV having a volumefraction of rubber particles greater than about 0.7 may have an elasticrecovery in the range from about 50% to 60% at 50% compression; to get ahigher elastic recovery one may modify the composition of the particularrubber dispersed in the continuous plastic phase, the ratio of thedispersed and continuous phases, the amounts and composition of thecuring agent(s) used, the amount of processing oil, and otheringredients, and other factors, with the expectation that, with enoughtrial and error, one can make a TPV with an elastic recovery in therange from about 60% to 65%. How these factors influence the morphologyof a TPV has been the subject of much study. Very little of this studyhas been devoted to identifying the key morphological requirement in aTPV which is most likely to provide much higher elastic recovery thanone would normally expect of the same TPV produced according to priorart procedures not specifically directed to the formation of thecritical morphology.

BACKGROUND OF THE INVENTION

Elastic recovery is that fraction of a given deformation that behaveselastically; a perfectly elastic material has a recovery of 100% while aperfectly plastic recovery has no elastic recovery. (see Whittington'sDictionary of Plastics 3rd Ed. 1993 Technomic Publishing). Elasticrecovery is an important property of a TPV which is expected to behavelike a natural rubber for examples in application where a TPV is used indynamic applications such as in hoses, and in sealing applications.

To date, a TPV is formulated with specified components including inaddition to the rubber and plastic, plasticizers, processing aids andfillers, by melt-blending the ingredients within generally definedprocessing parameters, until by trial and error, a usable TPV is made. A“usable TPV” is one which can be used in a marketable product. Inparticular, how the components are confined in a mixing andmelt-blending means, the rate at which mixing energy is inculcated, thetime over which the components are melt-blended, and the conditionsunder which the TPV is cooled are derived from experience and by trialand error. Though it is likely, with all the work directed to theproduction of TPVs over the past decade, that TPVs having optimummorphology may have been produced; but if they have been, the morphologyproduced was accidentally produced. An improvement in elastic recoverywas generally sought by varying the curing agent for the rubber, andalso the processing oil, processing aid, and filler. No one hasrecognized, much less identified, the critical morphological featuredirectly responsible for producing elastic recovery substantiallygreater than that which is obtained if the critical feature is lackingin a usable TPV.

A usable TPV, contains particles of rubber the majority of which, thatis greater than 50% by volume, are in the size range less than about 5μm, some being as large as 10 μm and others being as small as 0.1 μm orsmaller. Particles smaller than 0.1 μm are believed to be portionsfractured from larger particles while the TPV is being melt-blended, andthis very small size serves to define them as “very small” particles. ATPV preferred for its superior physical properties and acceptableelastic recovery has relatively large domains of rubber the majority ofwhich are in the size range from about 1-5 μm, preferably 1-3 μm, andthis size range serves to define them as “large particles”. The shape ofall particles resembles that of a distorted ellipsoid or elongatedovoid, and this shape is particularly evident in large particles. Theremaining rubber particles, in the size range larger than a “very small”particle and smaller than the mean diameter of the “large particles”,are defined as “small particles” or “mid-range particles” which also aregenerally ellipsoidal in shape. Because of the shape, the “diameter”referred to is the effective diameter, that is, the diameter theparticle would have had if it was spherical. The elongated ovoid shapeof the particles allows a high packing fraction of rubber particles in aunit volume of TPV, this being a characteristic of a usable TPV. Thenumber of very small particles is of minor consequence in a TPV; thenumber of small and large particles is not. To date, there has been noclear teaching as to what effect the size of the particles and theirdistribution has in a TPV particularly with respect to its elasticrecovery.

The morphology of various TPVs has been characterized in an articletitled Morphology of Elastomeric Alloys by Sabet Abdou-Sabet and RamanP. Patel (Rubber Chem. & Tech., Vol 64, No. 5,pg 769-779, Nov.-Dec.1991). Several variables affecting the morphology are identified,including the molecular weight of EPDM and PP; the ratio of EPDM to PP;degree of crosslinking; and types of crosslinks; but the effect of thethickness of a ligament, or the volume of continuous plastic phasebetween adjacent particles was not appreciated. The term “ligament” asused herein refers to the material of the continuous plastic phaseconnecting two adjacently disposed particles, and the “thickness of aligament” refers to the minimum narrowed distance between two adjacentparticles.

The origin of the overall elastomeric-like stress-strain behavior of aTPV including a large percentage of recoverable strain upon unloading isaddressed in publications by Kikuchi et al (1992), Kawabata et al (1992)and Soliman et al (1999). In an article titled Origin of RubberElasticity in Thermoplastic Elastomers Consisting of Crosslnked RubberParticles and Ductile Matrix, by Y. Kikuchi, T. Fukui, T. Okada and T.Inoue (Jour. of Appl. Polym. Sci. 50, 261-271 (1992), the strainrecovery of a TPE is analyzed using a two-dimensional model for atwo-phase system by finite element analysis (FEA). They concluded thatat highly deformed states at which almost the whole matrix has yieldedto stress concentration, the ligament matrix between rubber inclusionsin the stretching direction is locally preserved within an elastic limitand acts as an in situ formed adhesive for connecting the rubberparticles. They failed to appreciate that the thickness of the ligamentwas critically significant and that ligaments are deformed above theelastic limit, rather than preserved below it.

In an article titled Deformation Mechanism and Microstructure ofThermoplastic Elastomer Estimated On the Basis Of Its MechanicalBehavior under Finite Deformation, Sueo Kawabata, S. Kitawaki, et al.(Jour. of Appl. Polym. Sci. 50, 245-259 (1992), presented a model todescribe the large deformation mechanism of EPDM/PP and found that oildomains or layers between blocks play an important role in separatingthe rubber blocks from each other, allowing them to become free elementswithout friction between them. But Kawabata et al also failed torecognize the critical function of thin ligaments, less than 0.1 μmthick, between adjacent rubber particles, particularly large and smallrubber particles.

For simplicity the description relating to the critical thickness ofplural ligaments between a small particle and adjacent large particles,or, between large particles themselves, does not take into considerationcomponents other than the rubber particles and the continuous plasticphase in which they are dispersed. One skilled in the art will recognizethat such other components are typically dispersed between both phases,the relative amounts in each phase being determined by the particularcomposition of each phase and that of the other component. The presenceof such other components does not noticeably affect the criticality ofthe thickness of ligaments with respect to their effect on elasticrecovery.

SUMMARY OF THE INVENTION

It has been discovered that thin ligaments (as defined herein)connecting adjacent particles, and particularly “small” and “large”particles of rubber, is the critical determining factor which providessubstantially higher elastic recovery than obtained with ligamentsthicker than 0.1 μm; the mechanism of deformation and of elasticrecovery related to the microstructure and mechanical behavior of a TPVis simulated and confirmed by a micromechanical model of arepresentative volume element (RVE) in which key structural features,particularly ligament thickness and asymmetry, are systematicallyvaried; tensile properties are not significantly affected by a widerange of ligament thickness; in retrospect, having found what thecritical requirement is, parameters for rubber particles of anycomposition, and for any plastic phase, may be used in the model topredict the elastic recovery of the TPV.

It is therefore a general object of this invention to provide athermoplastic vulcanizate of an elastomer and a plastic comprisingparticles of elastomer dispersed in a continuous phase of plastic, suchthat a majority of particles, and particularly a majority of largeparticles which are present in a major proportion by volume relative tothe small particles, are adjacent at least one small particle criticallyspaced apart by ligaments, and at least 15% of the ligaments have athickness less than 10% of the mean diameter of large particles,preferably less than 5% of the “mean large particle diameter”, and theremaining ligaments have a thickness less than 50%, preferably from 15%to about 30% of the mean large particle diameter. Preferably the meanlarge particle diameter is in the range from 1 μm to 3 μm, mostpreferably about 1 μm; and, the small particle diameter is in the rangefrom 1% to 60% of the mean large particle diameter, preferably from 10%to 40%. Having confirmed the essential requirement for optimum, ornear-optimum elastic recovery for a specific TPV-R with amicromechanical model, it now permits one to predict what conditionswill generate the thin ligaments in any TPV with any other volumefraction and particle characteristics.

It is a specific object of this invention to provide a TPV having amajor proportion by volume of rubber relative to plastic (volumefraction>05) with a morphology in which the distribution of small andlarge particles is such that a small particle is proximately disposedrelative to at least 3 large particles; preferably the number of largeparticles is numerically smaller than the number of small and very smallparticles combined.

It is another specific object of this invention to modify a finiteelement analysis machine program to model a “five-particle” RVE(“5P-RVE”) which is uniquely adapted to mimic the mechanical behavior ofsmall and large rubber particles dispersed in a continuous plasticphase.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and additional objects and advantages of the inventionwill best be understood by reference to the following detaileddescription, accompanied with schematic illustrations of the prior artand a preferred embodiment of the invention, in which illustrations:

FIG. 1 is a TEM photomicrograph of a TPV in which the dispersed rubberparticles have a volume fraction of 0.77 at a magnification of 18,000 X

FIG. 2 is a schematic illustration of a simplified idealized finiteelement analysis (FEA) model of four large particles adjacent a smallparticle.

FIG. 3 is an example mesh 5-particle model of a representative TPV(“TPV-R”) having a Shore A 73 hardness.

FIGS. 4A-4F are finite element meshes (showing particles only) for sixmicromechanical models.

FIG. 5 shows curves predicting TPV-R plane strain stress-strain behaviorduring loading to a strain of −0.50 and unloading from that strain,based on the micromechanical model.

FIG. 6 shows curves predicting TPV-R plane strain stress-strain behaviorduring loading to a strain of −0.70 and unloading from that strain,based on the micromechanical model.

FIG. 7 shows curves for a RVE Case 6 model predictions of the planestrain stress-strain behavior compared to experimental results forloading to/unloading from a strain of −0.30.

FIGS. 8A-8D are deformed meshes during loading and unloading (imposedstrain of −0.70).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Preferred TPVs which benefit greatly from having high elastic recoveryin the range above 65%, preferably from about 70% to 95%, contain fromabout 55% to 95% volume fraction of rubber and have a hardness in therange from about Shore A 30 to Shore A 90. A major proportion of all thelarge particles in a preferred representative TPV, referred to as“TPV-R”, have a diameter in the range from about 1-3 μm The TEMphotomicrograph identified as FIG. 1 shows the morphology of the TPV-Rprepared in the illustrative example below. A sample of the TPV-R ismicrotomed and the surface stained with ruthenium tetroxide. The TEMimage is obtained with an efficient backscattered electron detectorwhich allows good differentiation between the phases and shows manydistorted ellipsoidal EPDM particles in the 1-3 μm size range separatedby regions of the continuous PP phase.

It is evident that the thickness of ligaments is relatively smallcompared to the diameter of even a small particle, and that there aremuch thicker regions of plastic matrix.

The TPV-R is prepared with a volume fraction Vf=0.77 of EPDM-rubber theremaining 0.23 being plastic matrix; this ratio is provided by about62.5 parts EPDM rubber and 37.5 parts PP, “parts” referring to parts byweight, ignoring other ingredients. Though EPDM may be made to include awide range of olefin and non-conjugated diene comonomers, the EPDM usedis a copolymer of ethylene, propylene and ethylidenenorbornene. Thethermoplastic polyolefin is general purpose polypropylene (PP), sp. gr.0.902, which is melt-blended with the rubber along with otheringredients specified in TPV #3 in Table X of U.S. Pat. No. 4,311,628 toAbdou-Sabet and Fath. The numbers following identification of aningredient refer to the parts by weight in the blend:

EPDM rubber 62.5; PP 37.5; Kaolin clay 23.1; Paraffinic extender oil78.13; magnesium oxide 0.13; titanium dioxide 3.23; zinc oxide 1.25;2,5-di(tert-amyl)hydroquinone 0.94; octylphenol/formaldehyde curingresin (Arofene 9273) 5.63. The ingredients are preferably melt-blendedin an extruder with a barrel temperature maintained in the range fromabout 160° C. to about 200° C., samples being taken from materialprocessed with a residence time ranging from about 2 min to 4 min, at atemperature ranging from about 170° C. to 180° C. The samples which hada hardness of about 73 Shore A, were tested for tensile and elasticrecovery and also prepared for TEM photomicrographs. The melt-blendedTPV has the following physical properties:

Hardness 72 Shore A 100% modulus, Kg/cm² 34.5 Ultimate Tens. Strength,Kg/cm² 86.5 Ultimate elongation, % 370 Tension set, % 11 Compressionset, % 17 ASTM #3 oil swell, % 87

In the TPV-R made with the above ingredients the volume fraction ofrubber particles is 0.77. Though there is a substantial amount of clay,it is believed that most of the clay is in the rubber particles, and theremainder, being inert in the plastic phase, does not affect theproperties of the PP so significantly as to affect the procedure used inthe micromechanical model.

The process for producing a TPV with substantially maximized elasticrecovery comprises melt-blending ingredients of the TPV at appropriateprocessing conditions for sufficient time while providing enough energyat a rate sufficient to produce a matrix of plastic in whichsubstantially all the large particles are adjacently disposed relativeto at least one small particle which is adjacent to at least 3 largeparticles, the small particle and the large particles being innear-contiguous relationship such that the small particle is spacedapart from at least one large particle by at least two ligaments havinga thickness less than 5% of the mean diameter of large particles in thematrix, and the remaining ligaments are in the range from more than 10%to about 50% of the mean large particle diameter. As will readily beevident, whether the TPV meets the critical criterion will be determinedby examination of a TEM photomicrograph. Upon making the determinationthat the criterion has been met, the melt-blending means is repetitivelyoperated to produce an arbitrarily large amount of TPV which has optimalelastic recovery.

The TEM photomicrograph, FIG. 1, is representative of a sample withabout 25% of the ligaments being critically thin; this sample showed thehighest elastic recovery, about 50% from 25% compression; and about 70%recovery from 50% compression. As will be evident from the Table 2presented below, these results are confirmed by FEA using a unique andnovel modification of commercially available Abaqus software asdescribed in greater detail below.

The elastomer phase was modeled using a three-dimensional RVE based on a5-particle model, and elastic properties measured for an EPDM rubberused in commercial Santoprene 73A rubber. The matrix phase was modeledusing elastic viscoplastic properties measured for PP with 20% by weightoil. The rubber particles were assumed to be round and distributed in ahexagonal close packing array. Five particles are modeled to simulatethe entire array using a plane strain finite element model in which thematerial being compressed is restrained between parallel planes, and thevolume fraction of rubber is the same in each. It is assumed that thereis no relative movement or “slip” at the boundary between a rubberparticle and the continuous PP phase because they are cohesively bonded,that is, one phase cannot be separated from the other without tearing itout.

Referring to FIG. 2 there is shown a closely packed array of a5-particle model the small particle in the middle surrounded by 4 largeparticles. In such an array there are 8 ligaments; a 4-particle arraywith a small particle surrounded by 3 large particles will have 6ligaments. The distribution of very small particles is ignored as theireffect is believed to be insubstantial and there is no definitive datato indicate otherwise.

The modelling of five particles in a matrix uniquely captures theinteraction of the particles with one another as well as the interactionwith the matrix. Some stages of deformation behavior are found to bematrix-dependent and other stages were particle-dependent. The5-particle model predicts both the loading and the unloading behaviorwhich was closely comparable to experimental results. Themicromechanical model revealed that the ligaments of PP plasticallydeform early on in the deformation and are the controlling factors inthe initial stiffness and “flow stress” of the system. The bulkierregions of PP are plastically essentially non-deformable; butdeformation of thin ligaments provides the elastomer-like behavior ofthe TPV. During unloading, the rubber undergoes recovery while thinligaments rotate and buckle to accommodate the recovery.

In the 5-particle model referred to generally by reference numeral 5P, acenter small particle RM is nested between four large particles R1, R2,R3 and R4 in the TPV-R. The plane strain nature of the model means thatthe particles are modelled as cylinders and the strain along the axis ofthe cylinder (i.e. into the paper) is restricted to be zero. Also notethat symmetry boundary conditions are applied to all edges of the modelsuch that vertical lines experience uniform horizontal displacement andhorizontal lines experience uniform vertical displacement. Because ofthe plane strain behavior of the model simulated stress-strain curvesare compared to experimental plane strain stress vs strain curves (asopposed to uniaxial stress vs. strain curves).

FIG. 3 depicts a typical finite element mesh for the five-particle RVE(5P-RVE) for the TPV-R. The 5P-RVE micromechanical model consists of acenter small particle nested between four larger particles. Note thatthe two-dimensional plane strain nature of the model means that the“particles” are cylinders where the cylinder axes are co-axial with theconstraint direction. The TPV-R particle volume fraction is 0.77 and theparticle area fraction (in the 1-2 plane) of the model is approximatedto be the volume fraction of particles (a reasonable approximation giventhe large volume fraction of particles and the thin ligaments of matrixbridging the particles). Symmetry conditions are imposed on all RVEboundaries such that initially vertical boundaries remain verticalu_(1|AB)=0 and u_(1|CD)=u_(1|C); and initially horizontal boundariesremain horizontal u_(2|BC)=0 and u_(2|AD)=u_(2|A).

In the 5P-RVE model the radius of the four large particles can beindependently set to permit the study the effect of varying particlesize distribution on the mechanical response. Six cases in particlegeometry are simulated; in all cases, the ratio of the radius of largeparticle R1 to small particle RM was set to 2.4(R1/RM=2.40). The tablebelow details the geometric configuration used in each simulation casein terms of the ratio of the radius of each large particle to that ofthe middle particle:

TABLE 1 Case 1 2 3 4 5 6 R1/RM 2.40 2.40 2.40 2.40 2.40 2.40 R2/RM 2.402.35 2.28 2.35 2.20 2.04 R3/RM 2.40 2.40 2.40 2.33 2.40 2.40 R4/RM 2.402.35 2.28 2.28 2.26 2.04

Because the oil-extended PP is similar to PP it is modelled using theconstitutive model for the rate-dependent elastic-viscoplastic behaviorof glassy polymers proposed in Boyce et al in an article titled LargeInelastic Deformation of Glassy Polymers: Part I: Rate-DependentConstitutive Model, in Mech. Matls 7, 15-33 (1988) as later modified byArruda and Boyce in Evolution of Plastic Anisotropy in Amorphous PolymerDuring Finite Straining, Intl. J. Plasticity, 9, 697-720 (1993).

The EPDM stress-strain behavior is modelled using the Arruda-Boyceconstitutive model for rubber elasticity. Each RVE is subjected to anaxial loading condition whereby the top edge is uniformly compressed inthe 2-direction The total force on edge AD, the RVE height H and the RVEwidth W are monitored as a function of applied displacement. The RVEtrue stress vs. strain response is then computed exactly as done in theexperiments.

Six micromechanical models of TPV-R are subjected to plane straincompressive loading and unloading to true strains of −0.50 and −0.70.FIG. 4 shows the finite element mesh (depicting only the particles) foreach of the six cases, the geometry of which is set forth in Table 1above. Case 1 possesses perfect symmetry with respect to therelationship of each large particle radius to that of the middleparticle. This sets up identical matrix ligament thicknesses betweeneach large particle and RM. For Cases 2 and 3, the asymmetry of the meshappears as a slightly smaller matrix ligament thickness bridging R1 andR3 with RM compared to the ligament thickness bridging R2 and R4 withRM. Case 4 possesses four different ligament lengths of similardimensions to those in Cases 2 and 3. Case 5 possesses even smallerligament thicknesses bridging R1 and R3 with RM than those of Cases 1,2, 3 and 4. Finally, Case 6 is seen to possess extremely thin ligamentsbetween two of the large particles and the middle particle. These sixcases permit the study of the influence of ligament length and asymmetryon the overall mechanical behavior and deformation mechanisms.

The results for these simulations will be shown by first comparing thestress-strain behaviors produced by each RVE. Four cases are thenselected (cases 1, 2, 5 and 6) and details regarding the deformation ofthe constituent phases are provided. The importance of geometry incontrolling various aspects of the mechanical behavior is thendiscussed.

FIGS. 5 and 6 depict the true stress-true strain behavior to strains of−0.50 and −0.70 respectively, as computed by the six micromechanicalmodels. As can be seen in the Figures, each model predicts a somewhatdifferent stress-strain behavior. Each model predicts a relatively stiffinitial response followed by a “flow stress” where the term “flowstress” refers to the stress level at which a dramatic roll-over ordecrease in slope occurs in the stress-strain curve. The flow stress isthen followed by strain stiffening/hardening (an increase in stress withcontinued straining). One can see that the initial stiffness is somewhatinfluenced by the geometry, where a smaller ligament length produces amore compliant initial response. The most dramatic effect of geometry isseen in the level of the flow stress. There is a dramatic decrease inflow stress with decreasing ligament thickness. The experimentallyobtained behavior is found to lie closest to the Case 6 prediction. FIG.7 depicts the Case 6 predictions together with the experimental data forloading and unloading to/from strains of −0.30 showing excellentagreement. This result indicates that it is the regions in the matrixmaterial with the thinnest ligament that control the overall deformationand thus the stress-strain behavior of the material. Case 6 is alsoobserved to most accurately predict the unloading (recovery) behavior ofthe material

Elastic Recovery from Compression (plane strain) vs. Ligament ThicknessData may be plotted from anal of various structures with equal volumeratio of elastomer and thermoplastic matrix The mean particle diameterof the large particles is 1.0 μm. The data are given in Table 2 below.

TABLE 2* Thinnest ligament % recovery from % recovery from Ex. (× 10⁻⁶m) 25% compression 50% compression 1 0.0597 25 20 2 0.0634 30 25 30.0458 32.8 30 4 0.0507 40 35 5 0.0403 70 60 6 0.0179 80 70 Actual TPV<0.02 83 74 *the foregoing data in italics are constructively presentedand rounded off as there was an apparent error in the programmingparameters.

The foregoing examples have varying symmetry and other geometricaldifferences that also act on the elastic recovery in addition to theeffect of the thinnest ligament. For example the Ex. 1 has a tightlyconstrained boundary condition that forces all of the plastic ligamentsto yield simultaneously and has a more severe effect than its ligamentthickness alone would indicate. Thus some of the numbers would appear tobe higher or lower than one would expect from familiarity with actualmaterial having the modulus of these theoretical materials.

It is evident from the above data that the effect of the thin ligamentsis demonstrated when it is less than about 0.0507 μm, or about 5% of themean large particle diameter. The higher the % compression, the greaterthe improvement in elastic recovery. Depending upon the compositions ofthe rubber and plastic phases, the number and diameters of large andsmall particles, and the volume fraction of rubber, it is evident that,for the specific rubber modelled, though there is improvement in elasticrecovery with thin ligaments less than 0.1 μm thick, the highestimprovement results with ligaments less than 0.05 μm.

The following illustrative examples demonstrate a dynamic representationof compression and recovery using a modified finite element analysispresented at the Gordon Research Conference on Elastomers Jul. 19, 1999at Colby-Sawyer College, New London, New Hampshire by Mary Boyce.

The model used herein is a 5 particle RVE that uses the Abaqus finiteelement analysis software. The rubber material is modeled using theArruda Boyce 8 chain model for rubber. The plastic matrix is modeledusing the elastic plastic theory mentioned earlier. Data for the rubberphase and plastic are measured separately using a Monsanto T-10tensometer and an Instron tensometer with controlled strain rate and fitto these theories. The rubber was mixed in a Brabender internal mixeradding the curative components on a rubber mill to keep the temperaturebelow the vulcanization temperature. Samples were press-cured to themaximum cure point based on a rheometer cure curve. The rubber toplastic volume ratio is selected based on a typical commerciallysignificant elastomeric material.

The following Table 3 sets forth data for theoretically formulated TPVsfor which the “% recovery” values in italics are constructivelypresented and rounded off. The ratio of rubber: plastic is 77:23 and ineach case the rubber is given a modulus of 025 MPa This ratio iscommercially significant and used in a soft rubbery material which isused in a wide variety of applications. The values generated for theTPVs compare a theoretical thermoplastic with varying stiffness,formulated with rubber properties maintained constant, on the assumptionthat the same rubber would be used in each recipe. The plastic stiffnessin the range from 100 MPa to 2500 MPa covers the range for mostcommercially useful thermoplastics. The three values selected are basedon the low, high and middle of the range. Only three of the severalexamples discussed previously are presented to span the ligamentthickness range.

TABLE 3 Medium Thinnest Low Modulus Modulus High Modulus Ligament TPV:100 MPa TPV: 500 MPa TPV: 2500 MPa Ex. (× 10⁻⁶ m) (% recovery) (%recovery) (% recovery) 1 0.0597 35 25 15 3 0.0458 65 32.8 30 6 0.0179 9580 60

The data in Table 3 demonstrate that the thinnest ligament exhibited thehighest elastic recovery irrespective of the stiffness of the plasticphase.

The following Table 4 sets forth the types of plastics used in recipesfor TPVs formulated to provide comparisons with actual rubber andplastic TPV compositions. The thermoplastic selected for tests arecommercially interesting materials used to formulate TPVs which it ishoped will have optimum elastic recovery.

TABLE 4 Thermoplastic Tested Grade Polyethylene (PE) 50:50 EscoreneLL6101/ Escorene LL1001 Nylon 6 (PA6) Capron 8202 Polybutyleneterephthalate (PBT) Valox 315

The following Table 5 presents recipes for rubber compounds used invarious samples. The natural rubber (NR) used was SMR CV60, the nitrilerubber (NBR) was Nipol 1022 and the ethylene acrylate rubber (AEM) wasVamac GLS.

TABLE 5 Ingredient NR NBR AEM SMR CV-60 100 Nipol 1022 100 Vamac GLS 100SP-1045 resin¹  5  5 SnCl₂.2H₂O  1  1 DOTG²  4 DIAK³ No. 1  1 ¹SP-1045resin is an alkyl pheol-formaldehyde resin ²di-orthotolyl guanidine³DIAK is hexamethylenediamine carbamate

The following specific TPV compositions are prepared, each having arubber:plastic ratio of 77:23 by volume: EPDM/PP; NR/PE; NBR/PA6; andAEM/PBT. Other additives that might be added in commercially usefulcompositions, additives such as, oils, plasticizers, stabilizers,fillers, reinforcements, etc. are purposely not included in the recipeto avoid complicating the problem of arriving at easily comprehensibleresults for these demonstration purposes.

The following Table 6 sets forth the values for % recovery calculatedfor the four TPV compositions specified above. The value in italics isare constructively arrived at and rounded off while some questionablecomputer results were resolved to complete the demonstration with thesevarious rubber/plastic pairs used for TPV composition.

TABLE 6 Calculated values for % recovery Thinnest Ligament EPDM/ NR/NBR/ AEM/ Ex. (× 10⁻⁶ m) PP PE PA6 PBT 1 0.0597 25 30.9 22 24.7 3 0.045832.8 40.2 31.7 34 6 0.0179 85 95 90 80 Real <0.02 83.0 94.1 88.6* 81.2**these measured values for real samples were prepared in a conventionalmanner as were the others, but appear to be substantially higher thancalculated indicating that the plastic phase was degraded.

Olefinic plastics are preferred, including polymers and copolymers oflower olefins having from 1 to 4 carbon atoms, one or more of whichmonomer may have a functional group, typically halogen, hydroxyl,carboxyl and copolymers of alpha unsaturated diolefins. If desired, athermoplastic a non-polyolefinic plastic may be used, for example, oneselected from the group consisting of polyamides, polycarbonates,polyesters, polysulfones, polylactones, polyacetals,acrylonitrile-butadiene-styrene (ABS), polyphenylene oxide (PPO),polyphenylene sulfide (PPS), styrene-acrylonitrile (SAN), polyimides,styrene-maleic anhydride (SMA) and aromatic polyketones, any of whichmay be used by itself or in combination with another. Most preferredengineering thermoplastic resins are polyamides and polyesters.Commercially available polyamides having a Tg or melting temperature(Tm) above 100° C. may be used but those having a Tm in the range from160° C. to about 280° C. are preferred. Preferred polyamides are nylon6, nylon 11, nylon 12, nylon 6,6, nylon 6,9, nylon 6,10, and nylon6/6,6. Most preferred are nylon 6, nylon 6,6, nylon 11, nylon 12 andmixtures or copolymers thereof. Additional examples of suitablepolyamides described in the Encyclopedia of Polymer Science andTechnology, by Kirk & Othmer, Second Edition, Vol. 11, pages 315-476,are incorporated by reference thereto as if fully set forth herein. Thepolyamides generally have a number average molecular weight of fromabout 10,000 to about 50,000, and desirably from about 30,000 to about40,0000.

Suitable thermoplastic polyesters include the various ester polymerssuch as polyester, copolyester, or polycarbonate, polybutyleneterephthalate (PBT), etc., a monofunctional epoxy endcapped derivativethereof, and mixtures thereof. The various polyesters can be eitheraromatic or aliphatic or combinations thereof and are generally directlyor indirectly derived from the reactions of diols such as glycols havinga total of from 2 to 6 carbon atoms and desirably from about 2 to about4 carbon atoms with aliphatic acids having a total of from 2 to 20carbon atoms and desirably from about 3 to about 15 or aromatic acidshaving a total of from about 8 to about 15 carbon atoms. Generally,aromatic polyesters are preferred such as polyethyleneterephthalate,polybutyleneterephthalate, polyethyleneisophthalate,polynaphthaleneterephthalate, and the like, as well as endcapped epoxyderivative thereof e.g., a monofunctional epoxypolybutyleneterephthalate. Various polycarbonates can also be utilizedand the same are esters of carbonic acid. A suitable polycarbonate isthat based on bisphenol A, i.e.poly(carbonyldioxy-4-phenyleneisopropylidene-1,4pheneylene).

The various ester polymers also include block polyesters such as thosecontaining at least one block of a polyester and at least one rubberyblock such as a polyether derived from glycols having from 2 to 6 carbonatoms, e.g., polyethylene glycol, or from alkylene oxides having from 2to 6 carbon atoms. A preferred block polyester ispolybutyleneterephthalate-b-polyethylene glycol which is available asHytrel from DuPont.

In addition to EPDM, or in lieu thereof other curable rubbers includehalogenated olefinic, acrylate and nitrile rubbers, and silicone.Rubbers useful in the blends include butyl rubber, halobutyl rubber, andEPR (ethylene/propylene rubber), arylonitrile/butadiene rubber (NBR) andnatural rubber. Combinations of two or more rubbers of different typescan also be used. Thermoplastic elastomers which can be successfullyfoamed by the process of the invention are described in the followingU.S. patents, the disclosures of which are herein incorporated byreference: U.S. Pat. Nos. 4,104,210; 4,130,534; 4,130,535; 4,299,931;and 4,311,628; inter alia. Also useful are blends of crystallinepolyolefin plastics and, partially cured rubbers, such as thosedescribed in U.S. Pat. Nos. 3,806,558 and 3,862,056, and blends ofcrystalline polyolefins and uncured EPR or EPDM rubber.

Copolymers of two or more of the following monomers may be used,provided at least one monomer has a functional group curable in acondensation reaction: an alkyl acrylate, a lower olefin, and anacrylate with a functional group.

In an alkyl acrylate, alkyl typically has 1 to 3 carbon atoms andincludes a repeating unit with a functional group and another repeatingunit chosen from ethyl acrylate, butyl acrylate, ethylhexyl acrylate,and the like. An olefinic repeating unit is chosen from an olefin havingfrom 2 to 4 carbon atoms, and the molar ratio of such olefin units toacrylate repeating units is typically in the range from 0.5 to 1.5. Apreferred functional group on an acrylic rubber is halogen, carboxyl,epoxy, or hydroxy. Suitable acrylate rubbers are commercially available.

The curing of a rubber in each stage is effected in the presence of aneffective amount of one or more curing agents present in an amountsufficient to result in a substantially complete cure of that rubber,namely at least 90 percent, though a lesser degree of cure, as low asabout 80 percent may be acceptable. Curing agents for a particularrubber are usually specified by the maker of the rubber.

Having thus provided a general discussion, described the essentialrequirement for making a TPV with optimum or near optimum elasticrecovery, and having confirmed the requirement with a micromechanicalmodel it is now possible to use that model to predict whether theessential requirement can be met in a TPV having any composition. Itwill be evident that the invention has provided an effective solution toa difficult problem. It is therefore to be understood that no unduerestrictions are to be imposed by reason of the specific embodimentsillustrated and discussed, and particularly that the invention is notrestricted to a slavish adherence to the details set forth herein.

We claim:
 1. A thermoplastic vulcanizate of an elastomer and a plasticcomprising particles of elastomer dispersed in a continuous phase ofplastic such that large particles which are present in a majorproportion by volume relative to small particles, are adjacent at leastone small particle spaced apart by ligaments, and at least 15% of theligaments have a thickness less than 10% of the mean diameter of largeparticles, the remaining ligaments having a thickness less than 50% ofthe large particles' mean diameter, wherein the mean large particlediameter is in the range from 1 μm to 3 μm, and the small particlediameter is in the range from about 1% to 60% of the mean large particlediameter.
 2. The TPV of claim 1 wherein said ligaments have a thicknessless than 5% of the mean large particle diameter, and said remainingligaments having a thickness from 15% to about 30% of said mean largeparticle diameter, and the small particle diameter is in the range from10% to 40% of the mean large particle diameter.
 3. The TPV of claim 1wherein said mean large particle diameter is about 1 μm.
 4. The TPV ofclaim 1 wherein said plastic is selected from the group consisting of apolyolefin, polyamide, polycarbonate, polyester, polysulfone,polylactone, polyacetal, acrylonitrile-butadiene-styrene (ABS),polyphenylene oxide (PPO), polyphenylene sulfide (PPS),styrene-acrylonitrile (SAN), polyimide, styrene-maleic anhydride (SMA)and aromatic polyketone, any of which may be used by itself or incombination with another.
 5. The TPV of claim 1 wherein said rubber isselected from the group consisting of EPDM rubber, halogenated olefinicrubber, EPR (ethylene/propylene rubber), acrylonitrile/butadiene rubber(NBR), natural rubber, silicone, and a copolymer of an alkyl acrylate, alower olefin, and an acrylate with a functional group.
 6. In a processfor melt-blending ingredients of a thermoplastic vulcanizate (“TPV”) toproduce a matrix of plastic in which large rubber particles having amean particle diameter in the range from 1 μm to 3 μm, and smallparticles having a mean diameter in the range from 1% to 60% of the meanlarge particle diameter, are randomly dispersed, the improvementcomprising, providing appropriate processing conditions for sufficienttime, including enough energy introduced at a rate sufficient to producea matrix of plastic in which substantially all said small particle isadjacently disposed relative to at least 3 large particles, the smallparticle and the large particles being in near-contiguous relationshipsuch that the small particle is spaced apart from at least one largeparticle by at least two ligaments having a thickness less than 5% ofthe mean diameter of large particles in the matrix, and the remainingligaments are in the range from more than 10% to about 50% of the meanlarge particle diameter, whereby said TPV has substantially optimumelastic recovery.